A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
U.S. Pat. No. 6,680,774 discloses a method and apparatus for mechanically masking a wafer to form exposure exclusion regions. Masking in this sense refers to blocking radiation. This is done using a mask which is opaque to the wavelength of radiation used by the lithographic apparatus. The term “mask” used in this context has a different meaning from the conventional meaning in lithography, and refers to device which is used to block radiation rather than to pattern radiation. The mask is arranged to overhang a predetermined exposure exclusion region of a photosensitive surface of a wafer. The mask is positioned over the wafer prior to exposure of the wafer, thereby shielding the regions of the photosensitive surface underlying the mask.
Also disclosed in U.S. Pat. No. 6,680,774 is a mask handling system having motorized base from which a beam extends. The beam may be raised, lowered and rotated. Mounted on the distal end of the beam are three arms. Each arm has a contact member, and these collectively engage a mask to load it into position on the wafer. However, in loading the mask into its correct position, the mask passes over the underlying wafer, thereby directly exposing regions of the photosensitive surface thereto, which may cause debris to fall onto the wafer and contaminate the photosensitive surface of the wafer.